The Great On-Turning

Tomorrow is the Day of the Great On-Turning – not of Deep Thought, but the Large Hadron Collider (LHC). It’s the sort of thing Stephen Hawking – who I can only hope will be there – once lamented not having in A Brief History of Time; twenty years ago he never dreamed the money would ever be available to build a machine this big and powerful. But here it is!

No doubt you’ve heard of this device from many other sources, not least Andrew Denton’s interview with Brian Cox on last night’s Enough Rope. Like most discussions of the Collider, that interview featured heavily the claims that the LHC will destroy the world, mostly fuelled by the ridiculous law suit still pending in the District Court of Hawaii (filed by a group of “concerned citizens”, at least one of whom has previously tried to stop other large particle collider projects). Well, we’ll have none of that here; if you’re still concerned by the warnings of crackpots, let CERN reassure you with their latest press release on the matter.

Instead, here’s a primer for those of you who are still unsure what it’s all about. First, let’s break down the name:

Large – the Large Hadron Collider is a “large collider of hadrons”, not a “collider of large hadrons” (hadrons do come in different types – see below – but not significantly different sizes). Some sources claim it’s the largest machine ever built by humans, and it’s certainly the largest science experiment – it’s a 27 kilometre long loop, buried underground near CERN in Geneva, and it crossed the border between Switzerland and France. It took 10 years to build with another 10 years of design work before that.

Hadron – a hadron is a particle made up of quarks, one of the fundamental particles that make up all matter. The most famous hadrons are baryons, which consist of three quarks, one of each “colour” – red, green and blue – that are held together by the strong nuclear force. Baryons include neutrons and protons, which make up the nuclei of atoms. The difference in charge between positively charged protons and neutrally charged neutrons is down to basic maths – different “flavours” of quark have different charges (it’s a little more complex than that, but we’re only interested in the quarks that make up “normal” baryons). In a neutron, the positive charge of one up quark (+2/3) is exactly balanced by the negative charge of two down quarks (2 x -1/3); in a proton, there are two up quarks (2 x 2/3) and one down quark (-1/3), resulting in a total charge of +1.

Collider – the LHC is a particle accelerator – it accelerates particles to very high velocities, giving them enormous energies. It’s also an “atom-smasher” (though it’s only smashing bits of atoms, not whole ones) – its purpose is to accelerate particles in two directions, colliding them together. It was collisions like this that allowed us to observe the existence of quarks, since normally they can’t exist on their own; smash some hadrons together, though, and their component bits go flying all around the place like bits of plastic bumper in a car crash.

The specific purpose of the LHC is to accelerate hadrons to speeds which will give them enough energy to simulate the state of matter only a few billionths of a second after the Big Bang, when things were very different to how they are now. This is hugely exciting because so little is known about the origins and initial formation of matter, or as Brian Cox put it, “what makes stuff stuff”.

One big question is to do with photons and W and Z bosons. A great success in particle physics was a combined theory explaining both the electromagnetic force and the weak nuclear force; it basically says they are two different aspects of the same force, which at high energies – like in the Big Bang – would manifest as a single “electroweak” force. The particles that carry these forces – photons and W and Z bosons, respectively – are different forms or states of the same particle, and at suitably high energies the combined force is carried by the Higgs boson. One of the big mysteries is why photons have no mass, while the W and Z bosons are massive (meaning they have mass, not that they’re huge!); hopefully observing the Higgs boson will shed some light on this!

It’ll be more than a month before the first collision is made – tomorrow’s “Great On-Turning” will involve only a single beam, not colliding with anything. But it’s an exciting time to be a scientist, or even a scientician – some big questions are going to get some kind of answer very soon. Of course, once the “answer” is determined, the real fun begins: trying to interpret what it all means… Perhaps it’s not so far from Deep Thought, after all.